feedback and use definition syntax

spec/Section 2 -- Language.md

@@ -288,8 +288,8 @@ There are three types of operations that GraphQL models:
 
 - query - a read-only fetch.
 - mutation - a write followed by a fetch.
-- subscription - a long-lived request that fetches data in response to source
-  events.
+- subscription - a long-lived request that fetches data in response to a
+  sequence of events over time.
 
 Each operation is represented by an optional operation name and a _selection
 set_.

spec/Section 6 -- Execution.md

@@ -164,7 +164,7 @@ ExecuteMutation(mutation, schema, variableValues, initialValue):
 
 ### Subscription
 
-If the operation is a subscription, the result is an event stream called the
+If the operation is a subscription, the result is an _event stream_ called the
 "Response Stream" where each event in the event stream is the result of
 executing the operation for each new event on an underlying "Source Stream".
 
@@ -217,18 +217,21 @@ chat room ID is the "topic" and each "publish" contains the sender and text.
 
 **Event Streams**
 
-An event stream represents a sequence of discrete emitted events over time which
-can be observed. As an example, a "Pub-Sub" system may produce an event stream
-when "subscribing to a topic", with an event emitted for each "publish" to that
-topic.
+:: An _event stream_ represents a sequence of events: discrete emitted values
+over time which can be observed. As an example, a "Pub-Sub" system may produce
+an _event stream_ when "subscribing to a topic", with an value emitted for each
+"publish" to that topic.
 
-Event streams may complete at any point, often because no further events will
-occur. Event streams may emit an infinite sequence of events, in which they may
-never complete. If an event stream encounters an error, it must complete with
-that error.
+An _event stream_ may complete at any point, often because no further events
+will occur. An _event stream_ may emit an infinite sequence of values, in which
+it may never complete. If an _event stream_ encounters an error, it must
+complete with that error.
 
-An observer may at any point decide to stop observing an event stream by
-cancelling it. When an event stream is cancelled, it must complete.
+An observer may at any point decide to stop observing an _event stream_ by
+cancelling it. When an _event stream_ is cancelled, it must complete.
+
+Internal user code also may cancel an _event stream_ for any reason, which would
+be observed as that _event stream_ completing.
 
 **Supporting Subscriptions at Scale**
 
@@ -254,8 +257,8 @@ service details should be chosen by the implementing service.
 
 #### Source Stream
 
-A Source Stream represents the sequence of events, each of which will trigger a
-GraphQL execution corresponding to that event. Like field value resolution, the
+A Source Stream is an _event stream_ representing a sequence of root values,
+each of which will trigger a GraphQL execution. Like field value resolution, the
 logic to create a Source Stream is application-specific.
 
 CreateSourceEventStream(subscription, schema, variableValues, initialValue):
@@ -280,7 +283,7 @@ CreateSourceEventStream(subscription, schema, variableValues, initialValue):
 ResolveFieldEventStream(subscriptionType, rootValue, fieldName, argumentValues):
 
 - Let {resolver} be the internal function provided by {subscriptionType} for
-  determining the resolved event stream of a subscription field named
+  determining the resolved _event stream_ of a subscription field named
   {fieldName}.
 - Return the result of calling {resolver}, providing {rootValue} and
   {argumentValues}.
@@ -291,24 +294,34 @@ operation type.
 
 #### Response Stream
 
-Each event in the underlying Source Stream triggers execution of the
-subscription _selection set_ using that event as a root value.
+Each event from the underlying Source Stream triggers execution of the
+subscription _selection set_ using that event's value as the {initialValue}.
 
 MapSourceToResponseEvent(sourceStream, subscription, schema, variableValues):
 
-- Let {responseStream} be a new event stream.
-- When {sourceStream} emits {event}:
+- Let {responseStream} be a new _event stream_.
+- When {sourceStream} emits {sourceValue}:
   - Let {response} be the result of running
-    {ExecuteSubscriptionEvent(subscription, schema, variableValues, event)}.
-  - Emit {response} on {responseStream}.
+    {ExecuteSubscriptionEvent(subscription, schema, variableValues,
+    sourceValue)}.
+  - If internal {error} was raised:
+    - Cancel {sourceStream}.
+    - Complete {responseStream} with {error}.
+  - Otherwise emit {response} on {responseStream}.
 - When {sourceStream} completes normally:
   - Complete {responseStream} normally.
 - When {sourceStream} completes with {error}:
   - Complete {responseStream} with {error}.
 - When {responseStream} is cancelled:
   - Cancel {sourceStream}.
+  - Complete {responseStream} normally.
 - Return {responseStream}.
 
+Note: Since {ExecuteSubscriptionEvent()} handles all _field error_, and _request
+error_ only occur during {CreateSourceEventStream()}, the only remaining error
+condition handled from {ExecuteSubscriptionEvent()} are internal exceptional
+errors not described by this specification.
+
 ExecuteSubscriptionEvent(subscription, schema, variableValues, initialValue):
 
 - Let {subscriptionType} be the root Subscription type in {schema}.